1. Field of the Invention
The present invention relates to a display panel for a cathode ray tube, and more particularly to a display panel for a cathode ray tube which has a panel structure approximate to a completely-flat panel structure in accordance with a correction of radiuses of curvature at inner and outer surfaces thereof while being capable of reducing a breakage thereof resulting from an in-furnace thermal impact in accordance with an optional variation in the compressive stress distribution exhibited therein.
2. Description of the Related Art
Referring to FIG. 1, an example of a typical cathode ray tube is illustrated. As shown in FIG. 1, the cathode ray tube includes a panel 10 mounted to a front portion of the cathode ray tube and made of a glass material, a shadow mask 12 arranged in rear of the panel 10 and adapted to allow electron beams to be accurately projected onto desired portions of a fluorescent film formed on an inner surface of the panel 10, and a frame 14 for supporting the shadow mask 12. The frame 14 is mounted to the panel 10 by means of stud pins 16 fixed to the panel 10 and springs 18 mounted to the frame 14. The springs 18 are coupled to the stud pins 16, respectively, thereby coupling the frame 14 to the panel 10. The cathode ray tube also includes a funnel 20 coupled to a rear end of the panel 10 at a front end thereof and adapted to maintain the interior of the cathode ray tube in a vacuum state, a cylindrical neck 22 connected to a rear end of the funnel 20 and made of a glass material, and an electron gun (not shown) fitted in the neck 22 and adapted to emit an electron beam. The cathode ray tube further includes an inner shield 26 mounted to a peripheral end of the frame 14 and adapted to shield an external magnetic field, a deflection yoke 28 mounted around the rear end of the funnel 20 and adapted to deflect the electron beam emitted from the electron gun, and a band 30 fitted around jointed portions of the panel 10 and funnel 20.
FIG. 2a illustrates the case in which the panel 10 has a panel structure for general screens. In this case, the panel structure of the panel 10 has a certain curvature at an outer surface thereof. FIG. 2a illustrates the case in which the panel 10 has a flat panel structure. In the case of FIG. 2b, the outer surface of the panel 10 is flat.
In either case, the panel 10 has, at the inner surface thereof, a face part 10a provided with a fluorescent film consisting of red, green, and blue dot trios of a fluorescent material to form an effective region for displaying an image, a central part 10b arranged at a central coordinate portion of the face part 10a, and a skirt part 10c arranged around the face part 10a. The skirt part 10c includes corner parts 10d and a seal edge part 10e coupled to the funnel 20.
In the general panel structure of FIG. 2a, an image displayed onto the screen is viewed in a convex state because of curved inner and outer surfaces of the panel. Furthermore, this panel structure also involves a diffused reflection of external light resulting in an increased fatigue of viewers.
The flat panel structure of FIG. 2b can eliminate the problems involved in the panel structure of FIG. 2a in that it is flat, thereby avoiding a phenomenon that an image displayed onto the screen is viewed in a convex state, and that it reduces the fatigue of viewers. However, this flat panel structure involves a thermal breakage of the panel resulting from an insurance of structural strength for the shadow mask.
To this end, in order to improve the surface strength of the panel 10 having the flat panel structure, a proposal has been made, in which a compressive stress layer is formed at the surface of the panel to avoid a thermal breakage of the panel due to heat generated during the manufacture of the cathode ray tube.
Meanwhile, a method has also been proposed, in which a high stress is temporarily generated at the panel 10. An example of such a method is a method for cooling the panel 10 to an annealing point or less. In accordance with this method, a thermal distribution is exhibited in the panel not only in a thickness direction, but also in a plane direction perpendicular to the thickness direction, due to a thermal distribution resulting from a three-dimensional structure of the panel and a cooling of the panel by air.
In particular, the cooling of the panel 10 at the corner parts 10d in accordance with a general cooling process tends to be carried out at a slow rate, as compared to the cooling of the panel 10 at the central part 10b, due to an influence of the three-dimensional structure of the panel 10.
In accordance with this process, a higher temperature gradient and a high stress are exhibited in the thickness direction at a higher cooling rate of the panel 10. Under this condition, the stress exhibited at the corner parts 10d of the panel 10 is less than that exhibited at the central part 10b. 
Accordingly, the panel 10, which is physically reinforced, exhibits a stress distribution in which the reinforced stress exhibited around each corner part 10d is lower than that exhibited at the central part 10b, and the reinforce stress exhibited at the inner surface of the face part 10a is lower than that exhibited at the outer surface of the face part 10a. Due to such a stress distribution, the panel 10 exhibits a degraded effect of preventing a thermal breakage from occurring during the manufacture of the cathode ray tube.
The conventional panel has a certain curvature at inner and outer surfaces thereof so that they have a desired structural strength, as shown in FIG. 2. By virtue of such a curvature, the panel also has, at each panel corner part 10d thereof, a thickness corresponding to 130% or less of the thickness of the central part 10b. 
As a result, the panel involve a greatly reduced in-furnace thermal breakage. In the case of a panel having a radius of curvature corresponding to 50,000 mm or more at the outer surface thereof while having a certain radius of curvature at the inner surface thereof, which is so called a xe2x80x9cflat panelxe2x80x9d, as shown in FIG. 2b, however, the thickness of each panel corner part 10d should be 170% or more of the thickness of the central part 10b in order to maximize the structural strength of the shadow mask 12. Due to such an abrupt increase in thickness, the panel 10 has a very undesirable structure in association with a breakage thereof, even though it makes it possible to maintain a desired strength of the shadow mask 12.
In order to solve this problem, it is necessary to considerably compress the surface of the panel 10. However, the in-furnace thermal breakage problem cannot be completely solved only using this method.
This is because an abrupt increase in thermal stress, which may result in an insolvable in-furnace thermal breakage is exhibited when the thickness difference, that is, the wedge rate, between the central part 10b and corner part 10d of the panel 10 is 230% or more. In the manufacture of a cathode ray tube, such a high thermal stress results in an in-furnace thermal breakage of the cathode ray tube. In order to minimize such a phenomenon, it is necessary to make a huge investment in order to achieve an improvement in furnace temperature. A great reduction in productivity is also involved, which results in a great increase in manufacturing costs.
The most effective method for preventing an in-furnace thermal breakage is to minimize the stress difference among the central part 10b, face part 10a, corner parts 10d, and seal edge part 10e of the panel 10.
Korean Patent Laid-open Publication No. 98-71757 discloses a technique in which compressive stresses are optionally provided at desired portions of a panel, respectively, so that the panel can be designed to have a reduced thickness while ensuring a security against explosions, in order to solve problems involved in conventional cathode ray tube designs in which a panel is designed to have an increased thickness at the face and peripheral parts thereof to achieve an enhancement in strength while ensuring a security against explosions.
However, there is no disclosure associated with schemes for providing a stress distribution capable of controlling an in-furnace breakage occurring in the manufacture of cathode ray tubes in the case using panels having an increased thickness, as in flat panels. Furthermore, where a high compressive stress of 16 MPa or mote is maintained, the stress difference between the central and corner parts of the panel is greatly increased due to the panel structure used. In this case, an in-furnace thermal breakage occurs very easily.
In order to obtain a panel structure having an appropriate stress distribution to exhibit a high resistance to a thermal breakage, accordingly, it is necessary to minimize the stress difference among the central part 10b, face part 10a, corner parts 10d, and seal edge part 10e of the panel 10.
Therefore, an object of the invention is to provide a display panel for a cathode ray tube which has a flat panel structure having, at face and skirt parts thereof, optionally-controlled compressive stresses respectively applied in accordance with a specific physical reinforcement scheme, thereby being capable of maximizing an effect of preventing an in-furnace thermal breakage of the panel.
In accordance with one aspect, the present invention provides a display panel for a cathode ray tube having an outer panel surface approximate to a complete flat surface, and an inner panel surface with a desired radius of curvature, wherein: a difference between a panel thickness at a central part of the panel and a panel thickness at each of diagonal corner parts of the panel is determined to satisfy a condition of xe2x80x9c1.7xe2x89xa6T2/T1xe2x89xa62.3xe2x80x9d, where, xe2x80x9cT1xe2x80x9d represents the panel thickness at the central panel part, and xe2x80x9cT2xe2x80x9d represents the panel thickness at the diagonal corner panel part; and compressive stresses exhibited at respective parts of the panel on the outer panel surface of the panel is determined to satisfy a condition of xe2x80x9c6.0 MPaxe2x89xa6|"sgr"|xe2x89xa615.0 MPaxe2x80x9d, where, xe2x80x9c"sgr"xe2x80x9d represents the compressive stresses exhibited at respective parts of the panel.
Preferably, the compressive stress exhibited at the central panel part is preferably determined to satisfy a condition of xe2x80x9c10.0 MPaxe2x89xa6|"sgr"C/C|xe2x89xa615.0 MPaxe2x80x9d, where, xe2x80x9c"sgr"C/Cxe2x80x9d represents the compressive stress exhibited at the central panel part. The compressive stress exhibited at a seal edge part of the panel is preferably determined to satisfy a condition of xe2x80x9c6.0 MPaxe2x89xa6|"sgr"S/E|xe2x89xa69.0 MPaxe2x80x9d, where, xe2x80x9c"sgr"S/Exe2x80x9d represents the compressive stress exhibited at the seal edge panel part. Preferably, the compressive stress exhibited at a seal edge part of the panel and the compressive stresses exhibited at respective portions of a face part of the panel extending in short-side and long-side directions are determined to satisfy conditions of xe2x80x9c0.8xe2x89xa6|"sgr"S/E/"sgr"Min|xe2x89xa61.4xe2x80x9d and xe2x80x9c0.8xe2x89xa6|"sgr"S/E/"sgr"Maj|xe2x89xa61.4xe2x80x9d, where, xe2x80x9c"sgr"S/Exe2x80x9d represents the compressive stress exhibited at the seal edge panel part, and xe2x80x9c"sgr"Minxe2x80x9d and xe2x80x9cMajxe2x80x9d represent respective compressive stresses exhibited at the short-side and long-side portions of the face panel part. Preferably, the compressive stress exhibited at a mold match line of the panel on the outer surface of the panel and those exhibited at respective portions of a face part of the panel extending in short-side and long-side directions are determined to satisfy conditions of xe2x80x9c0.35xe2x89xa6|"sgr"M/M/"sgr"Min|xe2x89xa60.65xe2x80x9d and xe2x80x9c0.35xe2x89xa6|"sgr"M/M/"sgr"Maj|xe2x89xa60.65xe2x80x9d, where, xe2x80x9c"sgr"M/Mxe2x80x9d represents the compressive stress exhibited at the mold match line of the panel on the outer surface of the panel, and xe2x80x9c"sgr"Minxe2x80x9d and xe2x80x9c"sgr"Majxe2x80x9d represent respective compressive stresses exhibited at the short-side and long-side portions of the face panel part. Membrane stresses exhibited at respective parts of the panel are preferably determined to satisfy a range from 30 kg/cm2 to 90 kg/cm2.
In accordance with another aspect, the present invention provides a display panel for a cathode ray tube having an outer panel surface approximate to a complete flat surface, and an inner panel surface with a desired radius of curvature, wherein: compressive stresses exhibited at respective parts of the panel on the outer panel surface of the panel in a state, in which the panel is assembled into a cathode ray tube, are determined to satisfy a condition of xe2x80x9c5.5 MPaxe2x89xa6|"sgr"|xe2x89xa612.5 MPaxe2x80x9d, where, xe2x80x9c"sgr"xe2x80x9d represents the compressive stresses exhibited at respective parts of the panel.
Preferably, the compressive stress exhibited at a central part of the panel is determined to satisfy a condition of xe2x80x9c9.0 MPaxe2x89xa6|"sgr"C/C|xe2x89xa612.5 MPaxe2x80x9d, where, xe2x80x9c"sgr"C/Cxe2x80x9d represents the compressive stress exhibited at the central panel part. The compressive stress exhibited at a seal edge part of the panel is preferably determined to satisfy a condition of xe2x80x9c5.5 MPaxe2x89xa6|"sgr"S/E|xe2x89xa68.5 MPaxe2x80x9d, where, xe2x80x9c"sgr"S/Exe2x80x9d represents the compressive stress exhibited at the seal edge panel part.